sparta::Buffer Enhancements

[bdutro]

Makes sparta::Buffer::erase() and makes sparta::Buffer::reverse_iterator behave more like their STL counterparts.

Note that there is a quirk in the behavior of reverse_iterator:

for(int i = 0; i < 10; ++i)
{
    buf.push_back(i);
}
auto it = buf.rbegin();
std::cout << *it; // Prints out 9
buf.pop_back();
std::cout << *it; // Prints out 8

Because sparta::Buffer::end() is a constant and sparta::Buffer::rbegin() points to end(), it will always be valid after an erase or pop_back as long as the buffer is not empty. This makes using pop_back while looping over reverse iterators unintuitive. However, the following pattern works as expected:

auto it = buf.rbegin();
while(it != buf.rend())
{
    it = buf.erase(it);
}

Fixes #464

[klingaard]

Awesome testing. Thank you!